The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Plasma processing systems are used in semiconductor fabrication. The system employs a plasma chamber that changes the electrical characteristics of a raw material (e.g. silicon) to manufacture semiconductor components. Examples of such components include transistors, medium and large state inductors, microprocessors, and random-access memory. The plasma chamber can perform sputtering, plasma etching, plasma deposition, and/or reactive ion etching in the manufacturing process.
In operation, the plasma chamber holds a semiconductor work-piece. Gas is then introduced into the plasma chamber at low pressure. An RF power generator applies RF power to the plasma chamber. The RF power changes the gas from a gaseous state to plasma. The plasma comprises electrically-charged ions that react with exposed regions of the semiconductor work-piece. A combination of these operations is performed on the work-piece to produce a particular semiconductor component.
Referring now to FIG. 1, portions of a typical plasma processing system 10 are shown. An RF power generator includes one or more output transistors 12. A direct current (DC) power supply provides power B+ to transistor 12. In some embodiments the DC power supply includes a switching power supply or power amplifier (PA). Output transistor 12 develops the RF power in accordance with an RF drive signal 14. The RF power communicates with an impedance matching network 16. An output of impedance matching network 16 communicates with an input of a plasma chamber 20, which typically has an input impedance of 50 ohms. Some installations include one or more dissipative bandpass filters 22 that connect in the feed line between transistor 12 and a load, shown as plasma chamber 20.
Transistor 12 typically generates the RF power at a single center frequency f0. During operation the input impedance of plasma chamber 20 varies continuously and spontaneously due to natural properties of the plasma. These input impedance variations cause drops in the power coupling efficiency between transistor 12 and plasma chamber 20. The input impedance variations can also cause RF energy to reflect from plasma chamber 20 back to transistor 12. The reflected RF energy can cause instability in the power delivery system and damage transistor 12. Filter 22 can be used to dissipate the reflected energy that occurs at frequencies outside of a passband of centered at center frequency f0. Examples of filters 22 are disclosed by Chawla et al. in U.S. Pat. No. 5,187,457, entitled “Harmonic and Subharmonic Filter”, assigned to the assignee of the present invention.
In applications where B+ is provided by a switching power supply the reflected RF energy can also cause the power supply to become unstable. Examples of filters 22 to address such situations are disclosed by Porter et al. in U.S. Pat. No. 5,747,935, entitled “Method and Apparatus for Stabilizing Switch Mode Powered RF Plasma Processing”.
Referring now to FIG. 2, a non-limiting example of a test measurement illustrates effects of the variations that the impedance of the plasma has on the stability of the system. A horizontal axis represents frequency over a range centrally located around a frequency f0. A vertical axis represents power coupled into plasma chamber 20. A peak at 44 represents desirable power coupling at center frequency f0. The plasma chamber input impedance variations contribute to peaks 46 at frequencies above center frequency f0 and peaks 48 at frequencies below center frequency f0. The peaks 46 and 48 indicate power at frequencies other than the fundamental frequency 44, such frequencies being generally undesirable. FIG. 3 depicts a Smith chart 50 illustrating the varying nature of the impedance match between transistor 12 and plasma chamber 20. A plot 51 of the impedance match crosses the real (horizontal) axis of Smith chart 50 at points 53 and 55. The crossings indicate resonance in chamber 20.
Referring now to FIG. 4 a non-limiting example of a test measurement illustrates an example of harmonic distortions that cause a loss of fidelity in the RF power over a frequency range. Fidelity generally refers to the undistorted propagation of the RF power from transistor 12 to the input of plasma chamber 20. The horizontal axis represents frequency, and the vertical axis represents power output by the power amplifier. A first peak 54 occurs at center frequency f0. A second peak 56, third peak 58, and fourth peak 60 occur at increasing integer multiples of center frequency f0. The amplitudes of second peak 56, third peak 58, and fourth peak 60 are progressively less than the amplitude of first peak 54. Energy at the second peak 56, third peak 58, and fourth peak 60 indicates that the RF power would be distorted at plasma chamber 20 without additional filtering. The fidelity is therefore less than ideal.